1. Technical Field
The present invention relates to an image projection apparatus for projecting images, a control method, a control program, and a storage medium storing the program.
2. Background Art
Image projection apparatuses known as projectors may employ a digital light processing (DLP: registered trademark) projector. The DLP projector employs a semiconductor device, called the digital micro-mirror device (DMD: registered trademark). having a large numbers of micro mirrors arranged in a matrix and used as light reflection faces whose tilt can be controlled within a given range of angles. As the light coming from a light source strikes the light reflection face of each of the micro mirrors of the DMD, the angle of the light reflection face of each of the micro mirrors is changed based on image signals, in which each of the micro mirrors can be set light-ON status (i.e., reflecting light to a screen) or light-OFF status (i.e., not reflecting light to the screen). In this configuration, the light coming from the light source can be selectively reflected to the screen by each of the micro mirrors to project a desired image on the screen. Further, a color wheel having three color filters, R (red), G (green), and B (blue), can filter light along its circumferential direction to project a color image. Specifically, the light coming from the light source is guided to the color wheel while rotating the color wheel having the three color filters, by which the light passes through the three color filters sequentially, and the R, G, and B lights are radiated on the DMD.
A problem with DLP projectors is that each DLP projector may have slightly different color reproduction properties (hereinafter, color properties) from any other. Therefore, even if the same image signal is input, different DLP projectors may project images with different color appearance and brightness. Thus, the same image signal may not be projected with the desired image quality.
Furthermore, a plurality of DLP projectors each having different color properties is concurrently used to provide a multi-screen display of a large scale. Therefore, the difference in color properties such as hue, value, and saturation between a plurality of images projected on the screen may be recognizable, and viewers feel the difference is unnatural and not acceptable. Because of variations in color properties of DLP projectors, methods of correcting the color properties of DLP projectors have been proposed.
Conventionally, images projected by a plurality of DLP projectors on a screen are captured by a camera for each of the DLP projectors to obtain color properties. The obtained color properties are then analyzed by comparison with a reference color property to determine relative differences. Based on that analysis, the color properties of projected images can be corrected.
For example, JP-2007-279643-A discloses a multi-screen display using a plurality of DLP projectors, in which a correction area is set for each screen, and a test pattern is projected on the multi-screen while changing color setting parameters for R, and B within a given range. Then, a camera captures an image of the test pattern at each correction area, and an average value of RGB of the image at each correction area captured by the camera is computed to conduct color correction. In this color correction, projection of the test pattern image, the capturing of the image of the test pattern, and the computing of the average value are repeated until the sum of the squares of differences between the computed average value of RGB and the reference value of RGB is minimized. Then, the color setting parameters of RGB satisfying a given acceptable condition are set to each of the DLP projectors.
Conventional color correction methods like that described above can correct differences in color properties among a plurality of DLP projectors. However, the methods may not handle occurrence of a false contour when time division half-toning control is used for the DLP projector. In the DLP projector, an angle of a light reflection face of each of the micro mirrors of the DMD can be switched between the light-ON status and the light-OFF status while being synchronized with a rotation of a color wheel and input image signals. The switching control (hereinafter, light-ON/OFF control) is conducted with time division to reproduce the colors of a projected image.
The half-toning expression, which expresses a color density level of the projected image, is conducted by a time division half-toning, which expresses half-toning by conducting the light-ON/OFF control for each of the micro mirrors using time division based on a desired image density.
The time division half-toning is conducted as follows. An output time of one image (one image frame) projected on a screen is divided into a light-ON time that sets the light-ON status for the micro mirrors, and a light-OFF time that sets the light-OFF status for the micro mirrors, and half-tone can be expressed by changing a ratio of the light-ON time and the light-OFF time. For example, if the first half of an output time of one frame is set as the light-ON time to generate white and the second half of the output time of one frame is set as the light-OFF time to generate black, the density of projected image becomes gray, which is a color intermediate between white and black.
However, when the time division half-toning control is conducted for the DLP projector, contour lines that do not exist in an original image (hereinafter, false contour) may appear in an area where brightness and/or color change gradually. The conventional color correction methods are not capable of preventing the occurrence of false contours.